Intermittent Fuel Starved 5.7
 

I guess the old addage that you can't fix it if it ain't broke holds true. Changed spark plugs, rotor and distributor cap. Also changed out the fuel/water separator cartridge. Fired right off on the muffs so we went to the lake. Ran like a top and pulled the grandkids on skis. Then pulled nephew out on slalom and the thing started cutting and coughing like it was out of fuel even though I had over 1/4 tank.
Pulled him over to the marina and added 12 gallons but no help. I let the thing sit for a while and got up on plane and then it took off like a banshee. After about 5 minutes same coughing and cutting out. It's like it's running rough but seems like fuel starved to me. Pretty sure it's not vapor lock and engine temp and oil pressure stay consistent.
I took it back out today and ran fine for about 15 minutes. Started same scenario all over again. The only thing I did related to fuel flow is change the filter. I primed it and it obviously is working. Could some crud have gotten knocked loose inside the line when I changed the filter? One thing that I did notice was the new filter (mercury) was shorter than the one that was on the boat (quicksilver). The diameter was identical so I can't imagine this being an issue. I'm pretty stumped here since it runs so well for a while then craps out. Thanks for any ideas or guidance.

Re: Intermittent Fuel Starved 5.7
 

Take the filter off and empty contents into a clear container. Let it sit to see if you have water in the fuel. The water will sink to the bottom.

Sent from my XT1254 using Tapatalk

Re: Intermittent Fuel Starved 5.7
 

OK the good news is the gas out of the filter looks crystal clear. Guess I didn't realize how much the dyes in gasoline have changed over the years. For those of us who grew up around tractors, the amber color of gasoline or the dark green of diesel were visible in the glass bowls at the bottom of the shutoff valve. Looks like a very light yellow color now.

The bad news is I still don't know what's causing the fuel delivery problem.

Re: Intermittent Fuel Starved 5.7
 

Pull the coil off and ohm it out across the primaries and then from the primaries to the output, secondary. The primaries (+ and -) should be zero but can have a little resistance. The secondary side will be in the 20k and lower range. Your problem sounds like a coil going bad. Hudson Steve can prolly give you the exact readings on your coil. He be smart like that.

Re: Intermittent Fuel Starved 5.7
 

OK, that's a great idea. I'll give that a shot and see what I find. Thank you for the info Brian!

Re: Intermittent Fuel Starved 5.7
 

Hey Scottie. Did the coil fix your problem? I have the same boat with the same problem.

Re: Intermittent Fuel Starved 5.7
 

I'll know more tonight when I can pull the coil off. I'll keep you posted on what I find.

Re: Intermittent Fuel Starved 5.7
 

I assume you have a Thunderbolt IV ignition system with the standard Mercruiser coil (392-7803A4). If so then the primary resistance should be 0.60-0.80 Ohms and the secondary resistance should be 9,400-11,700 Ohms.

Re: Intermittent Fuel Starved 5.7
 

Thanks for the info Hudson River Steve!

Re: Intermittent Fuel Starved 5.7
 

I toltd't y'all Steve would have the answer !!!